This application relates to and claims priority from Japanese Patent Application Nos. Hei. 9-198829 filed on Jul. 24, 1997, 9-201751 filed on Jul. 28, 1997, 9-204729 filed on Jul. 30, 1997, 9-255568 filed on Sep. 19, 1997 and 9-255569 filed on Sep. 19, 1997, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an electronic component mounted on a substrate board through an array of solder bumps attached to the back of the electronic component so that the electronic component is electrically connected to the substrate board, especially a mounting structure of the electronic component on the substrate board. The present invention is suitably applied to a mounting structure of an electronic component used in portable electronic equipments.
2. Related Art
As shown in FIG. 16, a conventional ball-grid array (hereinafter referred to as BGA) package 101 has an array of solder bumps 102 on its back surface, and a multi-layer printed wiring board 103 has a plurality of substantially circular electrodes 104. Each of the solder bumps 102 is melted and connected to each of the electrodes 104 so that the BGA package 101 is mounted on the multi-layer printed wiring board 103.
As shown in FIG. 17, the multi-layer printed wiring board 103 has a lead wire 105 extending from each of the electrodes 104. The lead wires 105 which extend from most externally-located electrodes 104a (hereinafter referred to as external electrodes 104a) are generally formed on the surface layer of the multi-layer printed wiring board 103; thereby facilitating external electrical connection of the extending wires 105. The external electrodes 104a are to be connected to the most externally-located solder bumps 102.
The lead wire 105 is covered with solder resist 106 for the purpose of protection. In FIG. 17, a slant line portion indicates an area covered by the solder resist 106. The entire surface of the electrode 104 (104a) is exposed so that a contact area between the solder bump 102 and the electrode 104 is increased. Therefore, in the external electrode 104a, the lead wire 105 is partially exposed because the lead wire 105 is formed on the surface layer of the multi-layer printed wiring board 103, and the solder bump 102 is thereby connected to both the external electrode 104a and a part of the lead wire 105.
When an external shock is applied to the BGA package 101 or the multi-layer printed wiring board 103, stress is applied to the external electrodes 104a intensively, especially when the multi-layer printed wiring board 103 is made of flexible resin or the like. Therefore, bonding portion between the external electrode 104a and the solder bump 102 is required to have sufficient bonding strength to withstand this intensive stress.
As shown in FIG. 17, the lead wire 105 from the external electrode 104a generally extends outwardly by the shortest route. That is, when a polygon is formed by connecting each of the centers of the adjacent external electrodes 104a, the lead wire 105 extends from a portion of the external electrode 104a located outside the polygon, in a direction in which the lead wire 105 extends away from a portion of the solder bump 102 located inside the polygon.
However, as shown in FIG. 16, the lead wire 105 extends from a portion A of the external electrodes 104a, which is located outside the polygon, and is mostly applied with the intensive stress when the external shock is applied to the BGA package 101 or the multi-layer printed wiring board 103. Therefore, when the external shock is applied, the solder bump 102 may be detached from the portion A, resulting in a contact failure between the BGA package 101 and the multi-layer printed wiring board 103.
In view of the foregoing problems, it is an object of the present invention to provide a mounting structure of an electronic component which is mounted on a substrate board in such a manner that a contact failure therebetween is prevented from occurring even when an external shock is applied thereto.
According to the present invention, an electronic component is mounted on a substrate board having a plurality of electrodes through a plurality of solder bumps. Most-externally-located electrode are provided with lead wires formed on a surface layer of the substrate board, extending from a portion of the corresponding electrode located inside of a polygon formed by connecting each of the centers of adjacent most-externally-located electrodes with respect to a plurality of the most-externally-located electrodes. As a result, the lead wire does not extend from an outside area of the polygon, to which stress is intensively applied due to an external shock. Thus, even when the external shock is applied to the electronic component or the substrate board, the solder bump is prevented from being detached from the most-externally-located electrode, thereby reducing contact failure between the electronic component and the substrate board.
Preferably, some of the most-externally-located electrodes are provided with lead wires embedded in the substrate board so that the solder bumps are bonded only to the most-externally-located electrodes, not to the lead wires. That is, the solder bump and the most-externally-located electrode are bonded to each other so that the solder bump to be bonded terminates at the outer peripheral portion of the electrode, i.e., at the side of the electrode. This increases bonding strength between the solder bumps and the most-externally-located electrodes, preventing the solder bumps from coming off the most-externally-located electrodes.
When the electronic component is provided with an interposer on which an array of plural electrodes is formed, the solder bumps are preferably bonded to most-externally-located electrodes disposed on the interposer of the electronic component so that the solder bumps are bonded to not only the surfaces but also the sides of the electrodes. This increases bonding strength between the solder bumps and the electrodes of the interposer, preventing contact failure between the solder bumps and the interposer.